The siphon is collecting 1,000 barrels of oil a day – roughly one-fifth of the oil leaking from the wellhead, by BP’s estimates, though some scientists suggest the amount of oil leaking in the Gulf oil spill could be much greater.

BP officials hope that, in coming days, the siphon system will be able to funnel more oil into tanker vessels on the surface. Moreover, they are proceeding with plans to try to stopper the wellhead by gumming it up with either a synthetic “mud” or bits of rubber tire and golf balls before capping the well with cement.

Defending dispersants

The EPA had refused to give the green light until it could test what effects the chemicals might have on marine ecosystems. Yet environmentalists have argued that the two dispersants BP is using are relatively toxic yet also relatively ineffective compared with other products in the market.

The EPA gave 12 other products better ratings on both scores, environmentalists note. But BP has stockpiled large amounts of the dispersant, called Corexit, and the EPA stresses that companies can choose whatever approved dispersant they choose.

EPA Administrator Lisa Jackson said it is too late to second-guess the products at hand and that anything is better than having the oil reach the coastal shores. “Until the flow of oil is stemmed, we must continue to take any responsible action that will reduce the impact of the spill, and that is what we are doing,” she said in a statement Monday.

Officials at a Robert, La., press conference Monday echoed that sentiment. The use of toxic dispersants below the surface of water “is all a series of tradeoffs,” said US Coast Guard Rear Adm. Mary Landry. Using dispersants below the surface allows for less use of dispersants overall by volume, said Landry.

The Coast Guard will continue to monitoring the process this week, she added.

“This was not done lightly.… We are certainly measuring and weighing what are the advantages,” she says.

Federal and BP officials were both on the defensive following weekend reports that plumes of oil were discovered thousands of feet below the surface. Inquiries among independent scientists inquired whether Corexit might be responsible.

The question is whether the oil is simply gathering beneath the surface or whether dispersants are breaking up oil molecules, leading them to sink and form undersea plumes.

Congressional caution

The uncertainty is giving some legislators pause. In a letter sent to Ms. Jackson Monday, Rep. Edward Markey (D) of Massachusetts wrote: “The release of hundreds of thousands of gallons of chemicals into the Gulf of Mexico could be an unprecedented, large, and aggressive experiment on our oceans.… The information regarding the chemical composition, efficacy, and toxicity of the dispersants currently being used is scarce.”

“There is no information to connect the use of dispersants to the subsurface layers they discovered,” wrote NOAA administrator Jane Lubchenco. [Editor's note:An earlier version of this story incorrectly identified NOAA and mischaracterized Dr. Lubchenco's position there.]

Further testing will be needed to verify the cause of the undersea oil clouds and what can be done to deal with them.

“We eagerly await results from their analyses and share … the goal of disseminating accurate information,” she wrote.